Setting an explicit scale when translating GeoTIFF to PNG

Question

When converting to PNG I'd like to convert the height of the GeoTIFF (which is a float) to UINT16, but also setting the maximum range to 5500, by clamping any values smaller or larger to min or max value. I'd also like to set it in such away that if I translate another file it has the exact same scaling factor.

For example

• range [130, 160] -> [630, 660]
• range [5, 200] -> [505, 700]
• range [-100, 6700] -> [400, 5500]

I have the following code, however, the scale doesn't seem to be consistent when I translate two adjacent GeoTIFFs to PNGs. There is discontinuity at the seems, due to (what I expect is) the scale for the two images being slightly different.

from osgeo import gdal, gdalconst

UINT_16_MAX_VALUE = 65535
UINT_16_MIN_VALUE = 0

def translate(output_file_name, dataset, top_left, bottom_right):

    # We need to change the scale of topological information as it is in Mean Sea Level. 
    opt = gdal.TranslateOptions(
        format='PNG',
        outputType=gdalconst.GDT_UInt16,
        # do we want nodata to be zero or smallest number or "unset"
        noData = None,
        # This assumes each pixels is 1 meter.
        projWin= [top_left[0], top_left[1], bottom_right[0], bottom_right[1]],
        # dem.tif comes in float values which are in meters. 
        # Assuming the original scale is a bit below sea level to a bit above. 
        scaleParams=[(-500,5000,UINT_16_MIN_VALUE, 5500)]
    )

    return gdal.Translate(output_file_name, dataset, options=opt)

Answer 1

Short Answer: I was setting the scale correctly, and I was just interpreting the output incorrectly.

Long Answer: There were two things that were causing me to think there was an issue with scaling. First, was in how I was viewing the data and second, was how I assumed values outside the scale were treated.

1. In QGIS, the software I was using to view the data, by default the viewer "stretches" the color scale to expose more details in your image. So if you are importing two different images with the same scale, the same height value might (and probably will) have a different color. What I did to "fix this issue" was in QGIS layer-symbology to manually set the min and max value across the DEM files to be the same. After doing this it was clear they were all on the same scale.

2. My other issue, which caused me to think there was an issue in the first place, is that values that are below the min and above the max are not clamped. I had incorrectly assumed that if you set the source scale 0-100 that any value above would be clamped to 100, then converted to the new scale. This is not the case, as @user30184 pointed out, the scale you set is linearly applied even for values outside the min and max of the output scale you provide.